This invention relates to a process for the preparation of isoprene by reacting isobutene with formaldehyde and recovering the formaldehyde.
According to a prior proposal isoprene may be prepared from isobutene and formaldehyde by first reacting isobutene, or a hydrocarbon fraction which contains isobutene, with aqueous formaldehyde in the presence of an acid catalyst to produce 4,4-dimethyl-m-dioxane, and then splitting this to form isoprene and formaldehyde in a second reaction stage carried out at an elevated temperature in the presence of an acid catalyst. Processes of this kind have been described, for example, in the following documents: Erdol und Kohle 15, pages 274-282 and 348-352 (1962), Z. vses. chim. Obsc. 14, 3, pages 313-319 (1969), DAS No. 1,271,106 DAS No. 1,233,880 and Belgian Pat. No. 735,564.
In these processes, aqueous phases which contain formaldehyde are obtained. To make these processes economical and obtain effluent water which can easily be clarified, the formaldehyde in the aqueous phases must be recovered as completely as possible. It would be particularly advantageous to be able to recover the formaldehyde in a form which is suitable for use in the first stage of the production of isoprene from isobutene and formaldehyde.
It is possible to recover the formaldehyde obtained from the second reaction stage of decomposition of 4,4-dimethyl-m-dioxane and it may, for example, be returned to the reaction between isobutene and formaldehyde to yield 4,4-dimethyl-m-dioxane (see DOS No. 1,618,331 and DOS No. 2,044,623). However, the dilute aqueous formaldehyde solutions which are obtained from the first reaction stage leading to 4,4-dimethyl-m-dioxane and which are heavily contaminated with organic constituents are very difficult to work up to recover the formaldehyde from them.
Various extraction processes, for example, have been proposed for working up these aqueous formaldehyde solutions, in particular to remove the organic constituents.
Thus, it has been disclosed in DOS No. 1,618,331 and DOS No. 2,044,623 that the aqueous phase from the first stage of the process can be extracted with the C.sub.4 -hydrocarbons (containing isobutene) used for the synthesis of 4,4-dimethyl-m-dioxane. Such an extraction does not remove the organic impurities from the aqueous formaldehyde phase sufficiently completely to enable the formaldehyde to be easily concentrated by distillation. At the same time, the extracted aqueous phase cannot be subjected to biological processes of waste water preparation without further purification.
Another extraction process has been disclosed in DT-PS No. 1,258,861 in which the aqueous phases obtained during the preparation of isoprene from isobutene and formaldehyde are purified. In this process, the aqueous phases obtained in the first stage and the second stage of isoprene synthesis are extracted separately with the unreacted C.sub.4 -hydrocarbons from the first stage of the reaction. It is stated in DT-PS No. 1,258,861 that the liquid remaining from the aqueous phase after extraction contains no organic compound apart from formaldehyde, so that it is ready for re-use as such or after concentration by evaporation. Nevertheless, it was found when repeating this process that the liquid phase left after extraction still contains certain quantities of organic substances apart from formaldehyde. It was not found possible to concentrate the formaldehyde in the extracted aqueous phases by distillation. Blockage of the distillation column invariably occurred owing to polymerisation of the organic compounds still left in the aqueous phase. The process according to DT-PS No. 1,258,861 is therefore by no means suitable for continuous recovery of the formaldehyde contained in the aqueous phases and re-use in the process for the preparation of isoprene. Moreover, the extracted aqueous phase which contains formaldehyde cannot be used for the biological preparation of effluent water without removal of the formaldehyde contained in it.
Another possible process for the recovery of formaldehyde is the concentration of dilute aqueous formaldehyde solutions, e.g. by distillation (J. F. Walker, Formaldehyde, Reinhold Publ. Co. N.Y. 1964). In such processes, the formaldehyde may either be obtained in a concentrated form as the sump product if distillation is carried out at reduced pressure or it can be obtained in a concentrated form as a distillate if distillation is carried out at an elevated pressure.
The aqueous formaldehyde phases obtained from the synthesis of isoprene, however, contain not only formaldehyde but also numerous other organic substances which would interfere with the distillation of formaldehyde.
Thus, formaldehyde can be obtained in a concentrated form as a sump product by vacuum distillation of these aqueous phases, but major proportion of the organic impurities are present in the distillate on account of their volatility so that the distillate cannot be discharged as effluent water.
Another possible method of working up formaldehyde by distillation consists of pressure distillation. As already described in DOS No. 1,618,331, however, in the case of pressure distillation the higher boiling organic compounds must be removed from the formaldehyde-containing aqueous phases, preferably by multi-stage evaporation, before distillation is carried out. Although an effluent water which is substantially free from organic compounds is obtained as a sump product when aqueous solutions containing formaldehyde are distilled under pressure, the high temperatures required for the distillation of formaldehyde-containing aqueous solutions under pressure result in resin formation even if the higher boiling organic compounds have been removed by distillation or extraction before pressure distillation is carried out. These resin-forming processes lead to blockage of the pressure distillation column within a short time.
It has therefore up to now not been possible to find a satisfactory means of working up the dilute formaldehyde-containing aqueous solutions obtained from the synthesis of isoprene in such a way that all the formaldehyde contained in the aqueous phases can be returned to the process of preparation of isoprene and in such a way that, at the same time, an effluent water is obtained which can be used for the biological preparation of effluent water. It would be desirable, however, to have a process available by means of which not only the formaldehyde-containing aqueous solutions from the second stage of the process, but also the more dilute aqueous formaldehyde solution obtained from the first stage when formaldehyde conversion is not quantitative can be treated for the recovery of formaldehyde. It is also desirable that any further dilute aqueous formaldehyde solutions obtained in the synthesis of isoprene can also be worked up.